In general, in a distributed database system called as a data store, for the purpose of acceleration of processing, data is distributively saved and managed in a plurality of node devices. Hereinafter, a node device to save the data is called a data storage node, which is described as a node.
As to which data is stored in which node, a pair of data identification information (assigned to each data) and a storage destination node are saved and managed by a server or a node (each having data management function).
In this distributed database system, a copy of the same data record is stored in a plurality of nodes. As a result, each node can send the data in response to a request from a client terminal or a system, respectively. Accordingly, a large number of accesses to the same data record can be distributively processed.
In this case, when a value of some data record stored in one node is updated, if the value of the same data record stored in another node is simultaneously updated, merit due to distribution of access is disturbed. Accordingly, this simultaneous update in another node is not performed. Usually, after a value of some data record in one node is updated, a value of the same data record stored in another node is replaced by orderly propagating the value from the one node using a protocol for data exchange.
Accordingly, when a value of one data record is updated (replaced) with the latest value, it takes a time to some extent until the value of this data record stored in all related nodes is updated with the latest value. As a result, before the value of this data record stored in all nodes is updated, a client terminal or a system accessing this data record possibly acquires old value (not the latest value).
Furthermore, when an application acquires the value of the data record while this data record is being updated, even if the value is updated just after acquisition of the value, the application cannot know this updated value.